The present invention pertains to devices such as shale shakers wherein a screen which lies generally horizontally is moved in a repetitive manner, i.e. vibrated or oscillated, such movement typically involving at least some motion transverse to the long dimensions of the screen, i.e. generally vertically. Relatively large solids, such as cuttings from a well, are supported on the screen and, by virtue of its repetitive movements and/or angle, moved therealong from an inlet end to an outlet end, while a carrier liquid and more finely divided solids, e.g. drilling mud, are separated from these cuttings, falling downward through the interstices of the screen, to be separately collected, further treated, and recycled.
A major problem with such devices is that the very material being handled may tend to dampen or otherwise interfere with proper movement of the screen so that the material is not properly distributed or moved by the screen. Typically, this dampening tends to occur at the inlet end of the screen, which is usually the very place where proper movement and distribution is most needed.
The typical approach to this problem has been to mount the screen at an angle so that, while it still lies generally horizontal, i.e. with a major component of direction in the horizontal sense, it is inclined so as to adjust its center of gravity to compensate for the dampening of the screen movement at the inlet end. However, in the past, this has been largely a matter of guesswork; it is imprecise, requires stopping and restarting of operation of the device, and is a general nuisance. Furthermore, if the density of the material changes, the screen may then have to be readjusted, multiplying the above problems.
Other problems generally encountered in well drilling operations, while seemingly unrelated to the aforementioned problems with shale shakers, can, surprisingly, also be addressed by preferred embodiments of the present invention, which solves the basic shale shaker problem. This involves early detection and remedying of such conditions as: gas cutting of the drilling mud; lost circulation; changes in volumetric flow rate of mud; changes in drilling rate; and changes in the proportions of cuttings to mud. It will be appreciated that some of these, such as lost circulation, are always problems in and of themselves. Others may or may not be problems in themselves, but in any event, may be symptomatic of those which are definite problems. For example, a change in the volumetric flow rate of mud could be symptomatic of partial circulation loss.